There have been proposed various types of low-temperature show cases, each having an opening which is proved for reaching goods therein and is closed with transparent doors and the like, as described in Japanese Utility Model Publication No. 60-1976. Such a so-called reach-in type show case generally employs a forced cold-air circulation system to cool the goods-storing rooms, in which substantially parallel streams of cold air forms an air curtain running, behind the openings, from the upper to the lower portion of the front openings, thereby fulfilling cooling the rooms and, at the same time, shutting the ambient air out of the room even when the front doors are opened. Such air curtains are useful in preventing the rooms to warm up.
The air curtain of the type as disclosed in said Publication, however, has the same or less width compared with the width of the front opening.
Although the cold air is effused uniformly from the outlet, it has a non-uniform transverse distribution of fluid velocity, which is smallest near the side walls due to viscous resistance along the side walls of a show case, and increases towards its center. Such non-uniformity causes disadvantage that the ambient air tends to flow into the case from the sides of the air curtain because the air velocity is low at the edges and the curtain is weak.
Such disadvantage is encountered not only with reach-in type show cases but also so-called open type show cases such as one disclosed in Japanese Patent Publication No. 60-14988, because both ends of an air curtain still contacts with the inner surface of the side walls of the show case.
Such weakness of the air curtain would not be very serious and would not result in warming up of the storing room if the front doors are opened only for a very short period for taking out some goods. However, when the doors are opened for a long period for selecting particular goods or replenishing goods, the ambient air will flow into the storing room undesirably from the side of the air curtain, so that the storing room will be warmed up. This is a more serious problem always encountered with open type show cases than with reach-in type show cases.
A prior art low-temperature show case of this type is also disclosed in Japanese Patent Publication No. 60-14988. FIG. 2 of this Publication shows an art of making holes in rising steps provided on the ceiling board forming a part of the passage of the cold air, said holes also given in the depending portions of the steps. The holes are intended to introduce flows of the cold air into the storing room so as to maintain the temperature of the ceiling board near the holes above the dew point in the storing room and prevent formation of dews on the lower surface of the ceiling board.
In order to provide a greater amount of cold air in the storing room, the depending portion must appreciably extend downwardly. However, this decreases effective volume of the storing room undesirably, since the height of the ceiling must be lowered with reference to the uppermost shelf at least by the vertical size of the holes in the depending portion. This volume loss is not negligible especially as the number of the steps is increased.
On the other hand, if the holes are formed very small in size for the purpose of restoring the height of the ceiling board, the holes may no longer prevent the dews effectively, since then such small holes cannot induce the cold air into the storing room but instead withdraws (or ejects) air from the storing room.
Conventionally, a heating system of this type of show case generally halts its operation during defrosting the ice and frost formed on the refrigeration apparatus as seen in, for example, Japanese Utility Model Early Publication No. 62-80178, which discloses a radiation heater for defrosting installed beneath the refrigeration apparatus, and a conduit inclined towards the drain port of the case provided under the heater for receiving melt frost. Japanese Utility Model Early Publication No. 46-33836 discloses an electric heater mounted on the circumference of the refrigeration apparatus, said heater having a portion protruding downwardly and extending nearly to the drain port.
However, since the heater of the former Publication is arranged beneath the refrigeration apparatus, extending substantially in parallel with the bottom of the refrigeration apparatus, pieces of ice or frost fallen from the refrigeration apparatus tends to accumulate near the drain port located far from the heater and is likely to remain not melted, since the drain port receives less heat than the conduit. Consequently, the remaining ice and frost will continually grow there as the electricity is turned off for defrosting. The growing ice will eventually clog the drain port and paralyze it, which has been a serious disadvantage.
The heater of the latter Publication, on the other hand, can defrost the drain port by means of said protruding portion, but fails to provide sufficient radiation heat uniformly to the frost receiver. Further, the radiation heat is not enough to melt the remaining ice fallen off the evaporator.
Japanese Patent Publication No. 62-1518 discloses a low-temperature show case having a partition wall that extends from the rear wall to the front for partitioning the goods storing room into an upper and a lower storing case, the former of which has a front opening and is cooled by forced circulation of cold air from a refrigeration apparatus, while the latter has an upper opening and is cooled by forced circulation of cold air from a refrigeration apparatus. This show case is reported capable of effectively removing dews generated in the upper storage room during refrigeration.
In order to illuminate the lower storage rooms, most show cases, including the ones mentioned above, utilizes fluorescent lamps installed between the upper and lower storage rooms, or beneath the partition walls. Usually, door-storage sections are formed under the partition walls for storing slidable doors for the opening, and these lamps are often mounted at corners, in general at the front ends, of the storage portions. The presence of such lamp affects the flow of the cold air. The cooled doors cools the door-storage section while they are housed in the storage section. Since the ambient air can easily flow into such door storage section, the region is liable to have significant dews formed therein, which is often. The heat of the lamp, however, is not sufficient to remove the dews formed deep in the storage section. Therefore, some heating means such as an extra heater is needed for removing the dews.
Low-temperature show cases of this type have further difficulty in that, although the heat conduction across the windowpanes is not great, the outer surfaces of the transparent windows are eventually cooled from inside and covered with dews formed thereon as the ambient air contacts the windows. The dews will come down on the surfaces of the transparent windows, and drop onto the floor or onto parts provided in the cases. These dews causes the metal surfaces of the parts to rust or become sources of stain. Hence, most show cases are provided with means for treating such dews, as disclosed in Japanese Utility Model Early Publication Nos. 58-159483 and 62-143175. The former of these Publication teaches a method to remove the dews formed on the transparent windows by leading them into the cold-air passage of the upper storage room. The latter teaches a method of removing the dews formed on the doors by leading them into the cooling chamber.
Although thermally insulated drainage means are provided for leading into said passage the dews formed on the transparent windows, the dews tend to freeze on the internal surface of the inner walls or frost on the portion of the refrigeration apparatus closer to the goods storing room because the dews are cooled by the cold air while they are led into the drainage means through the cold-air passage in the upper storage room. This undesirably causes rather prompt clogging of the passage. Still further, since the cold-air passage of the upper storage room is connected with the lower cold-air passage via the drain passage, dews formed in the upper storage room undesirably tends to frost on the refrigeration apparatus connected with the lower cold-air passage.
On the other hand, the latter has a disadvantage that the dews led into the cooling chamber helps the frost formed on the refrigeration apparatus grow.